Ornithophily

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Hummingbird Phaethornis longirostris on an Etlingera inflorescence Phaethornis longirostris.jpg
Hummingbird Phaethornis longirostris on an Etlingera inflorescence

Ornithophily or bird pollination is the pollination of flowering plants by birds. This sometimes (but not always) coevolutionary association is derived from insect pollination (entomophily) and is particularly well developed in some parts of the world, especially in the tropics, Southern Africa, and on some island chains. [1] The association involves several distinctive plant adaptations forming a "pollination syndrome". The plants typically have colourful, often red, flowers with long tubular structures holding ample nectar and orientations of the stamen and stigma that ensure contact with the pollinator. Birds involved in ornithophily tend to be specialist nectarivores with brushy tongues and long bills, that are either capable of hovering flight or light enough to perch on the flower structures.

Contents

Plant adaptations

A lesser violetear Colibri-thalassinus-001-edit.jpg
A lesser violetear
Inflorescences of Butea allow birds to perch on the stalk ErythrinaFlower.jpg
Inflorescences of Butea allow birds to perch on the stalk

Plant adaptations for ornithophily can be grouped primarily into those that attract and facilitate pollen transfer by birds, and those that exclude other groups, primarily insects, [4] protecting against 'theft' of nectar and pollen. [5] The ovules of bird flowers also tend to have adaptations that protect them from damage during vigorous foraging by hard bird bills. [6]

One of the general adaptation patterns is the red flower color for many plant species. The pollinator, birds, are tetrachromats, and one type of the single cone has specific opsin to detect long-wavelength light (below about 600 nm). [7] Therefore, birds have red photoreceptors and are sensitive to red color, and red flower colors can have a strong contrast with green leaf background. Better bird attraction is thus the primary reason behind the red color adaptation. Moreover, flowers of generalist bird-pollinated species from the New World have purer red colors than those from the Old World, which often have a secondary reflectance peak around shorter-wavelength region. The secondary peak reduces the contrast of red color with the background and the avoidance of insects (higher risk of nectar robbing), decreasing the efficiency of pollination. [8] [9]

The flowers of generalist bird-pollinated plant species differ from those pollinated by specialized birds, such as hummingbirds or sunbirds by lacking long corolla tubes and having brush-like, exserted stamens. [10] Most bird pollinated flowers are red and have a lot of nectar. They also tend to be unscented. [11] Flowers with generalist pollinators tend to have dilute nectar but those that have specialist pollinators such as hummingbirds or sunbirds tend to have more concentrated nectar. [12] [13] The nectar of ornithophilous flowers vary in the sugar composition, with hexoses being high in passerine pollinated species while those that are insect pollinated tend to be sucrose rich. Hummingbird pollinated flowers however tend to be sucrose rich. [14]

Different plants have also developed specific adaptations for bird pollination. Many plants of the family Loranthaceae have explosive flowers that shower pollen on a bird that forages near it. They are associated mainly with flowerpeckers in the family Dicaeidae. [15] In Australia, some species of Banksia have flowers that open in response to bird actions thereby reducing the wastage of pollen. [16] In tropical dry forests in southern India, ornithophilous flowers were found to bloom mainly in the hot dry season. [17] Calceolaria uniflora , a species of Scrophularaceae from South America, has a special fleshy appendage on the lower lip of the flower that is rich in sugar. This is fed on by the least seedsnipe (Thinocorus rumicivorus) and in the process the birds brush pollen onto their head and transfer them to other flowers. [18]

The rat's tail babiana ( Babiana ringens ) produces a strong stalk within the inflorescence that serves as a perch for the malachite sunbird as it visits the flower. [19] Heliconias have special sticky threads that help in the adhesion of pollen to smooth structures such as the bill of a hummingbird. [20] Some African orchids of the genus Disa have pollinaria that stick to the feet of visiting sunbirds. [21]

Plants need to protect against nectar and pollen being taken by non-pollinators. [22] Such animals are sometimes classified as thieves, which simply remove resources without pollinating, and robbers, which damage the flower to access resources. Flowers specialized for pollination by long-billed birds may be especially vulnerable to theft. [23] For example, some bees and birds that cannot reach down the long tubes of bird pollinated flowers simply pierce the flower at the base to obtain nectar, without pollinating. [24]

Bird adaptations

Ruby-throated hummingbird (Archilochus colubris) at scarlet beebalm flowers (Monarda didyma) RubyThroatedHummingbird.jpg
Ruby-throated hummingbird (Archilochus colubris) at scarlet beebalm flowers ( Monarda didyma )

The main families of specialized nectar feeding birds that are involved in ornithophily are the hummingbirds (Trochilidae), sunbirds (Nectariniidae), and the honey-eaters (Meliphagidae). Other important bird groups include those in the families the Icteridae, the honeycreepers (Thraupidae, Carduelinae), white-eyes (Zosteropidae) and the South African sugar-birds (Promeropidae). Birds may obtain nectar either by perching or by hovering with the latter mainly found in the hummingbirds and sunbirds. Within the hummingbirds, two kinds of foraging are noted with non-territorial "hermit" hummingbirds which forage longer distances and the territorial non-hermits. [5]

Hummingbirds have the ability to digest sucrose unlike many passerines that prefer hexoses (fructose and glucose). Starlings and their relatives will completely avoid sucrose. [25] Nectar feeding birds typically have a mechanism to quickly excrete excess water. They may have to drink four to five times their body mass of liquid during the day to obtain enough energy. [26] Hummingbirds are capable of excreting nitrogenous wastes as ammonia since they can afford more water loss than birds that feed on low-moisture food sources. [27] [28] Hummingbirds and sunbirds also have special anatomical and physiological adaptations that allow them to quickly excrete excess water. Hummingbirds are also able to turn off their kidney function at night. [29]

In some birds such as white-eyes, the pollen dusted by the plants on the forehead of the birds may increase the wear of these feathers leading to increased moulting and replacement. [30]

Patterns in the evolution of ornithophily

About 7000 neotropical plant species are hummingbird-pollinated [31] in contrast to about 129 species of North American plants that have evolved ornithophilous associations. [32] Nearly a fourth of the 900 species of the genus Salvia are bird-pollinated in Central and South America and a few also occur in South Africa. [33] Tropical China and the adjacent Indochinese countries harbor relatively few bird-pollinated flowers, among them is Rhodoleia championii , a member of the family Hamamelidaceae, which at any one site can be visited and pollinated by up to seven species of nectar-foraging birds, including Japanese white-eyes (Zosterops japonicus, Zosteropidae) and fork-tailed sunbirds (Aethopyga christinae, Nectariniidae). [34]

Hummingbirds rely on nectar for energy, and ornithophilous flowers need hummingbirds’ assistance with pollination in order to reproduce. While the birds are feeding, pollen sticks to their beaks, which will rub off on the next flower they visit, pollinating it. [35] Over time, the co-dependence on one another causes the co-evolution of pollination syndromes. [36] For example, different species of hummingbirds have differently shaped beaks, presumably to allow them to drink nectar from the flowers around them. [37] It is widely believed that short-billed hummingbirds drink from wider flowers with short petals, and hummingbirds with longer bills have close relationships with flowers with long, narrow corollas. Most of the time, long-billed species have access to both short and long flowers, but they often avoid short flowers to avoid competition. [38] Further, hummingbirds with curved bills will forage at straight-petaled flowers, but  straight-billed birds are less likely to visit curved flowers. [39] Ornithophilous flowers pollinated by hummingbirds often have reproductive structures that are vertically oriented. This creates a favorable upright body position for hummingbirds during feeding, one that allows them to sufficiently flap their wings for hovering. [40] Hummingbirds prefer to visit larger and taller floral displays, and it has been proven and confirmed through many studies that birds prefer flowers with red or pink petals over other colors. [41]

Bird pollination is considered as a costly strategy for plants and it evolves only where there are particular benefits for the plant. [42] High altitude ecosystems that lack insect pollinators, those in dry regions or isolated islands tend to favour the evolution of ornithophily, most by specialized nectarivorous birds, such as hummingbirds or sunbirds. [5] Plants pollinated by generalist birds are most diverse in tropical and subtropical lowlands with a pronounced climatic seasonality. These plants are mostly large, woody species that produce a large number of open flowers at the same time in contrast to the mostly small shrubs and herbs that are pollinated by specialized nectarivorous birds. Since generalist bird-pollinated plants are mostly self-incompatible they needed to adapt to pollinators that mostly provide outcrossing, such as generalist birds. These birds mostly feed on arthropods, fruits or seeds even if much nectar is available and therefore move a lot through the forest. By this activity they often move between nectar-providing plants and provide outcrossing. [43] Generalist bird-pollinated plants even evolved deterring mechanisms against specialized nectarivorous birds and bees since these groups tend to establish feeding territories within one tree and thus most conduct self-pollination.

On islands however, generalist bird pollination did not evolve to avoid self-pollination but adapted to a reliable pollinator since bees and butterflies are rare just as on montane forests. [43]

Migration and flowering synchrony

Time of flowering is often used to mark the start of spring in temperate climate zones. Recently, studies have consistently found that plants respond to increasing temperatures by flowering earlier. [44] Strong coevolution between hummingbirds and flowers has led to an adaptive specialization outcome in which important behaviors of both hummingbirds and flowers become synchronized. Because hummingbirds depend heavily on nectar, it is very possible that their migration is correlated with the time of flowering of flower species. [45] For specialist hummingbird species, flowering phenology is extremely important for survival during and following fall migration. For example, the migratory route of the S. rufus is linked to the florescence of a unique collection of flower species. [46] S. rufus prefer S. iodantha flowers. [47] [46] Studies have found that the presence of S. rufus is coupled with the flowering of S. iodantha in specific locations. Time of flowering is therefore significant for the survival of S. rufus during fall migration. Similarly, peak flowering of Impatiens capensis flowers corresponds to the peak migration time of the Ruby-Throated Hummingbird. [46]

Other associations

Several mite species (mainly in the genera Proctolaelaps , Tropicoseius and Rhinoseius , family Ascidae) have evolved a phoretic mode of life, climbing into the nostrils of hummingbirds that visit flowers and hitching a ride to other flowers where they can feed on the nectar. Hummingbird flower mites favour plants in the families of Heliconiaceae, Costaceae, Zingiberaceae, Amaryllidaceae, Rubiaceae, Apocynaceae, Bromeliaceae, Gesneriaceae, Lobeliaceae and Ericaceae, members of which are associated with hummingbirds. [48]

See also

Related Research Articles

<span class="mw-page-title-main">Hummingbird</span> Family of birds

Hummingbirds are birds native to the Americas and comprise the biological family Trochilidae. With approximately 366 species and 113 genera, they occur from Alaska to Tierra del Fuego, but most species are found in Central and South America. As of 2024, 21 hummingbird species are listed as endangered or critically endangered, with numerous species declining in population.

<span class="mw-page-title-main">Pollinator</span> Animal that moves pollen from the male anther of a flower to the female stigma

A pollinator is an animal that moves pollen from the male anther of a flower to the female stigma of a flower. This helps to bring about fertilization of the ovules in the flower by the male gametes from the pollen grains.

<span class="mw-page-title-main">Hoverfly</span> Family of insects

Hoverflies, also called flower flies or syrphids, make up the insect family Syrphidae. As their common name suggests, they are often seen hovering or nectaring at flowers; the adults of many species feed mainly on nectar and pollen, while the larvae (maggots) eat a wide range of foods. In some species, the larvae are saprotrophs, eating decaying plant and animal matter in the soil or in ponds and streams. In other species, the larvae are insectivores and prey on aphids, thrips, and other plant-sucking insects.

<span class="mw-page-title-main">Coevolution</span> Two or more species influencing each others evolution

In biology, coevolution occurs when two or more species reciprocally affect each other's evolution through the process of natural selection. The term sometimes is used for two traits in the same species affecting each other's evolution, as well as gene-culture coevolution.

<span class="mw-page-title-main">Pollination</span> Biological process occurring in plants

Pollination is the transfer of pollen from an anther of a plant to the stigma of a plant, later enabling fertilisation and the production of seeds, most often by an animal or by wind. Pollinating agents can be animals such as insects, for example beetles or butterflies; birds, and bats; water; wind; and even plants themselves. Pollinating animals travel from plant to plant carrying pollen on their bodies in a vital interaction that allows the transfer of genetic material critical to the reproductive system of most flowering plants. When self-pollination occurs within a closed flower. Pollination often occurs within a species. When pollination occurs between species, it can produce hybrid offspring in nature and in plant breeding work.

<span class="mw-page-title-main">Sunbird</span> Family of birds

Sunbirds and spiderhunters make up the family Nectariniidae of passerine birds. They are small, slender passerines from the Old World, usually with downward-curved bills. Many are brightly coloured, often with iridescent feathers, particularly in the males. Many species also have especially long tail feathers. Their range extends through most of Africa to the Middle East, South Asia, South-east Asia and southern China, to Indonesia, New Guinea and northern Australia. Species diversity is highest in equatorial regions.

<i>Heliconia</i> Genus of plants

Heliconia is a genus of flowering plants in the monotypic family Heliconiaceae. Most of the 194 known species are native to the tropical Americas, but a few are indigenous to certain islands of the western Pacific and Maluku in Indonesia. Many species of Heliconia are found in the tropical forests of these regions. Most species are listed as either vulnerable or data deficient by the IUCN Red List of threatened species. Several species are widely cultivated as ornamentals, and a few are naturalized in Florida, Gambia, and Thailand.

Parallel evolution is the similar development of a trait in distinct species that are not closely related, but share a similar original trait in response to similar evolutionary pressure.

<i>Babiana ringens</i> Species of flowering plant

Babiana ringens, the rat's tail, is a flowering plant endemic to Cape Province of South Africa. The foliage is long and erect with an inflorescence consisting of a sterile main stalk adapted for ornithophily, pollination by birds. The plant bears bright red, tubular flowers on side branches close to the ground. It is a perennial that grows in nutrient-poor sandy soil and flowers during the winter rains.

<span class="mw-page-title-main">Entomophily</span> Form of pollination by insects

Entomophily or insect pollination is a form of pollination whereby pollen of plants, especially but not only of flowering plants, is distributed by insects. Flowers pollinated by insects typically advertise themselves with bright colours, sometimes with conspicuous patterns leading to rewards of pollen and nectar; they may also have an attractive scent which in some cases mimics insect pheromones. Insect pollinators such as bees have adaptations for their role, such as lapping or sucking mouthparts to take in nectar, and in some species also pollen baskets on their hind legs. This required the coevolution of insects and flowering plants in the development of pollination behaviour by the insects and pollination mechanisms by the flowers, benefiting both groups. Both the size and the density of a population are known to affect pollination and subsequent reproductive performance.

<span class="mw-page-title-main">Zoophily</span> Pollination by animals

Zoophily, or zoogamy, is a form of pollination whereby pollen is transferred by animals, usually by invertebrates but in some cases vertebrates, particularly birds and bats, but also by other animals. Zoophilous species frequently have evolved mechanisms to make themselves more appealing to the particular type of pollinator, e.g. brightly colored or scented flowers, nectar, and appealing shapes and patterns. These plant-animal relationships are often mutually beneficial because of the food source provided in exchange for pollination.

<span class="mw-page-title-main">Nectar</span> Sugar-rich liquid produced by many flowering plants, that attracts pollinators and insects

Nectar is a sugar-rich liquid produced by plants in glands called nectaries or nectarines, either within the flowers with which it attracts pollinating animals, or by extrafloral nectaries, which provide a nutrient source to animal mutualists, which in turn provide herbivore protection. Common nectar-consuming pollinators include mosquitoes, hoverflies, wasps, bees, butterflies and moths, hummingbirds, honeyeaters and bats. Nectar plays a crucial role in the foraging economics and evolution of nectar-eating species; for example, nectar foraging behavior is largely responsible for the divergent evolution of the African honey bee, A. m. scutellata and the western honey bee.

<span class="mw-page-title-main">Nectarivore</span> Animal in which nectar is a main source of nutrition in their diet

In zoology, a nectarivore is an animal which derives its energy and nutrient requirements from a diet consisting mainly or exclusively of the sugar-rich nectar produced by flowering plants.

<span class="mw-page-title-main">Palynivore</span> Group of herbivorous animals

In zoology, a palynivore /pəˈlɪnəvɔːɹ/, meaning "pollen eater" is an herbivorous animal which selectively eats the nutrient-rich pollen produced by angiosperms and gymnosperms. Most true palynivores are insects or mites. The category in its strictest application includes most bees, and a few kinds of wasps, as pollen is often the only solid food consumed by all life stages in these insects. However, the category can be extended to include more diverse species. For example, palynivorous mites and thrips typically feed on the liquid content of the pollen grains without actually consuming the exine, or the solid portion of the grain. Additionally, the list is expanded greatly if one takes into consideration species where either the larval or adult stage feeds on pollen, but not both. There are other wasps which are in this category, as well as many beetles, flies, butterflies, and moths. One such example of a bee species that only consumes pollen in its larval stage is the Apis mellifera carnica. There is a vast array of insects that will feed opportunistically on pollen, as will various birds, orb-weaving spiders and other nectarivores.

<span class="mw-page-title-main">Pollination syndrome</span> Flower traits that attract pollinators

Pollination syndromes are suites of flower traits that have evolved in response to natural selection imposed by different pollen vectors, which can be abiotic or biotic, such as birds, bees, flies, and so forth through a process called pollinator-mediated selection. These traits include flower shape, size, colour, odour, reward type and amount, nectar composition, timing of flowering, etc. For example, tubular red flowers with copious nectar often attract birds; foul smelling flowers attract carrion flies or beetles, etc.

<span class="mw-page-title-main">Malachite sunbird</span> Species of bird

The malachite sunbird is a small nectarivorous bird found from the highlands of Ethiopia southwards to South Africa. They pollinate many flowering plants, particularly those with long corolla tubes, in the Fynbos.

<span class="mw-page-title-main">Nectar robbing</span> Foraging behavior

Nectar robbing is a foraging behavior used by some organisms that feed on floral nectar, carried out by feeding from holes bitten in flowers, rather than by entering through the flowers' natural openings. Nectar robbers usually feed in this way, avoiding contact with the floral reproductive structures, and therefore do not facilitate plant reproduction via pollination. Because many species that act as pollinators also act as nectar robbers, nectar robbing is considered to be a form of exploitation of plant-pollinator mutualism. While there is variation in the dependency on nectar for robber species, most species rob facultatively.

<span class="mw-page-title-main">Trap-lining</span> Feeding strategy amongst certain families of birds

In ethology and behavioral ecology, trap-lining or traplining is a feeding strategy in which an individual visits food sources on a regular, repeatable sequence, much as trappers check their lines of traps. Traplining is usually seen in species foraging for floral resources. This involves a specified route in which the individual traverses in the same order repeatedly to check specific plants for flowers that hold nectar, even over long distances. Trap-lining has been described in several taxa, including bees, butterflies, tamarins, bats, rats, and hummingbirds and tropical fruit-eating mammals such as opossums, capuchins and kinkajous. Traplining is used to term the method in which bumblebees and hummingbirds go about collecting nectar, and consequently, pollinating each plant they visit. The term "traplining" was originally coined by Daniel Janzen, although the concept was discussed by Charles Darwin and Nikolaas Tinbergen.

<span class="mw-page-title-main">Flower constancy</span> Tendency to visit certain flower species

Flower constancy or pollinator constancy is the tendency of individual pollinators to exclusively visit certain flower species or morphs within a species, bypassing other available flower species that could potentially contain more nectar. This type of foraging behavior puts selective pressures on floral traits in a process called pollinator-mediated selection. Flower constancy is different from other types of insect specialization such as innate preferences for certain colors or flower types, or the tendency of pollinators to visit the most rewarding and abundant flowers.

<span class="mw-page-title-main">Pollinator-mediated selection</span> Process in which pollenators effects a plants evolution

Pollinator-mediated selection is an evolutionary process occurring in flowering plants, in which the foraging behavior of pollinators differentially selects for certain floral traits. Flowering plant are a diverse group of plants that produce seeds. Their seeds differ from those of gymnosperms in that they are enclosed within a fruit. These plants display a wide range of diversity when it comes to the phenotypic characteristics of their flowers, which attracts a variety of pollinators that participate in biotic interactions with the plant. Since many plants rely on pollen vectors, their interactions with them influence floral traits and also favor efficiency since many vectors are searching for floral rewards like pollen and nectar. Examples of pollinator-mediated selected traits could be those involving the size, shape, color and odor of flowers, corolla tube length and width, size of inflorescence, floral rewards and amount, nectar guides, and phenology. Since these types of traits are likely to be involved in attracting pollinators, they may very well be the result of selection by the pollinators themselves.

References

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